Origin and evolution of nephrites, diopsidites and giant diopside crystals from the contact zones of the Pounamu Ultramafics, Westland, New Zealand

Abstract

ABSTRACT Diopside-rich rocks (diopsidites) are interlaminated with nephrite in boulders derived from metasomatic contacts developed between Pounamu Ultramafic meta-serpentinite and country rock Alpine Schist, Westland, New Zealand. Petrographic textures indicate that parental tremolite rock, formed by metasomatic diffusion during metamorphism, has been intensely deformed and recrystallised to alternating semi-nephrite and nephrite domains during development of a secondary crenulation cleavage. Nephrites are subsequently sequentially overprinted by porphyroblastic tremolite, diopside, then further tremolite. Crystallisation is controlled by fluctuating activities of SiO2, CaO and H2O in associated fluids. Pervasive dissolution of nephritic tremolite and crystallisation of diopside generates diopsidites containing accessory epidote, uvarovite and zincian chromite formed in equilibrium with H2O-rich fluids. Diopsidites are in turn overgrown by coarse grained (in places > 50 cm long) diopside crystals, interpreted to have infilled an extension fracture that formed during ongoing uplift of the Southern Alps.